In my opinion, the best coin to invest for long-term is always bitcoin. In addition, I think ethereum have a great potential to reach good prices in the future.
BTC already too old and sluggish compared to many other altcoins. People still buy it because they new in crypto, don't know nothing except BTC
I don't understand that why you called bitcoin sluggish. Of course there are a lot of altcoins to invest, but we are talking about the coins which can be thought as long term investments.